Abstract
Software-defined radios (SDR) are gradually becoming a practical option for implementing RF communication systems due to their low cost, off-the-shelf availability, and flexibility. Although the analog limitations of the hardware devices in these systems create barriers to some applications, creative algorithms in digital signal processing (DSP) can improve the results. In some cases, this improvement is essential to establishing a robust and reliable communication. The universal software radio peripheral (USRP) is a popular hardware that can be used alongside the SDR. Among many capabilities of USRP and its changeable daughter boards is receiving GPS signals. The GPS satellites transmit data on two main frequencies, L1 (1575.42 MHz) and L2 (1227.60 MHz). In this chapter, the focus is on describing a detailed implementation of the real-time DSP-based algorithm for USRP to detect GPS signal, namely the L1 band that transmits at 1575.42 MHz.
TopIntroduction
This chapter details the steps taken to apply DSP algorithms to USRP. One should be able to follow these steps with the specified set of hardware and software to get the same configuration on the proper USRP device. These projects were implemented under the supervision of NASA scientists and faculty advisors in dedicated NASA labs. Following lessons learned here and authors’ previous experiences in data visualization and signal processing research and training relevant labs were designed to enhance the Computer Engineering program at the Virginia State University (VSU) (Sheybani, 1992, 2002, 2006, 2007, 2008, 2010, 2011, 2012, 2013, 2017; Javidi, 2008, 2010, 2014, 2015, 2017; Ouyang, 2010; Garcia-Otero, 2011; Badombena-Wanta, 2010; Ettus, 2014, 2015; Luttamaguzi, 2017; Mathworks, 2014).
The overall goal of this project is to retrieve the carrier signal of the L1 band from a GPS satellite signal using USRP. The USRP uses a daughterboard to tune to the radio frequency of the receiver interface or the transmitter interface. Daughter boards are selected based on the frequencies involved in the project. In many cases, the selection of an RF daughterboard is made solely on the application requirements for frequency coverage. Figure 1 shows the ranges of different daughter boards (Anon, 2014).
Key Terms in this Chapter
SDR: It is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.
DSP: It is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The signals processed in this manner are a sequence of numbers that represent samples of a continuous variable in a domain such as time, space, or frequency.
USRP: It is a range of software-defined radios designed and sold by Ettus Research and its parent company, National Instruments. Developed by a team led by Matt Ettus, the USRP product family is intended to be a comparatively inexpensive hardware platform for software radio, and is commonly used by research labs, universities, and hobbyists.